FIG. 17 is a view showing a conventional substrate processing apparatus. As shown in FIG. 17, the substrate processing apparatus includes a substrate holder 101 for holding a substrate W, and a processing chamber 102 for holding a processing liquid therein. The processing chamber 102 includes an inner chamber 103 in which the substrate W, held by the substrate holder 101, is disposed, and an outer chamber 104 surrounding the inner chamber 103. The outer chamber 104 is coupled to the inner chamber 103 through a liquid delivery pipe 105. One end of the liquid delivery pipe 105 is coupled to a bottom of the outer chamber 104, and other end of the liquid delivery pipe 105 is coupled to a bottom of the inner chamber 103. A pump P is coupled to the liquid delivery pipe 105.
When the pump P is operated, the processing liquid is sucked from the bottom of the outer chamber 104 through the liquid delivery pipe 105, and is supplied through the liquid delivery pipe 105 into the inner chamber 103. The processing liquid ascends in the inner chamber 103 to form an ascending flow, and overflows a side wall of the inner chamber 103 into the outer chamber 104. Further, the processing liquid is returned to the inner chamber 103 through the liquid delivery pipe 105 with the operation of the pump P. In this manner, the processing liquid circulates between the inner chamber 103 and the outer chamber 104 while forming the ascending flow in the inner chamber 103.
In order to increase a processing rate (which is also referred to as a processing speed) of the substrate W, it is necessary to increase a flow velocity of the processing liquid (i.e., a flow velocity of the processing liquid ascending in the inner chamber 103) which is in contact with a surface of the substrate W. However, when the flow velocity of the processing liquid in the inner chamber 103 is increased, the processing liquid may spout from the inner chamber 103 as shown in FIG. 17, or the substrate holder 101 may be floated up by the ascending flow of the processing liquid. Therefore, it is difficult to increase the flow velocity of the processing liquid in the inner chamber 103.
On the other hand, when the processing liquid flows in the inner chamber 103 at a low flow velocity, the processing liquid forms a laminar flow in the inner chamber 103. If the substrate W is immersed in the laminar flow of the processing liquid, the substrate W may be locally processed, and may be non-uniformly processed.